Why doesn't hydrogen peroxide feel warm when it is put on a cut?

The amount of energy releases is the determining factor if you feel heat. For example, moss decomposing a rock has an exothermic reaction occurring, just not very fast and not releasing a lot of energy all at once. Fire burning a piece of would is an extremely fast reaction that releases a lot of energy all at once. That energy is what you feel as heat. The problem with hydrogen peroxide is that it more than likely doesn’t release a lot of energy very quickly. Therefore it may not seem hot to you. It is a common misconception that it is a good antiseptic. Most people believe that the foaming is a big reaction cleaning out a cut. It is actually the hydrogen peroxide reacting with the catalase in your blood cells and just breaking down into oxygen and bubbling up through the liquid.

Wow that was an amazing answer. Thank you for that.

I’m glad I could help.

And sweet avatar by the way. The old command and conquer, you can’t beat classics like that.

Thanks, and welcome to Fluther.

@sentry why do you suggest its not a good antiseptic. It is a powerful oxidizing agent and this is what kills the bacteria. Although it can cause slight tissue it can be used to stop bleeding in small capillaries and as an oral debriding agent.

Yeah, it’s supposed to be quite the antiseptic, considering how cheap it is. I heard the oxygen gas it releases while decomposing is what kills the bacteria which can’t handle it.

It is true that it does stop some bleeding, but it will only stop small vessels that are slowly bleeding. The hydrogen peroxide you buy at walgreens or any drug store is actually only 3% hydrogen peroxide. That means that it is 97% water. When hydrogen peroxide is added to a cut, it does in fact kill all the bacteria on the surface of the wound, as well as all the cells that are trying to heal your cut. Hydrogen peroxide never makes it inside your cut to clean/harm you because of its reaction with catalase which turns it into water and oxygen. Hopes this help answer you guys questions.

The endothermic process of evaporation exceeds the exothermic process of the chemical reaction, thus it feels cool.

I agree, it is not a good antiseptic – it actually damages tissue as well as being an ineffective bacteriostatic.

Thanks, everyone, especially hossman, great, simple answer.

Sentry, I would modify your answer slightly. Exogenous peroxide is ineffective, but endogenous peroxide is a very effective endogenous antiseptic. It might surprise some to learn that human neutrophils (a type of white blood cell) produce both hydrogen peroxide and hypochloric acid (bleach) from the combined activities of the enzymes phagocyte oxidase, superoxide dismutase and myeloperoxidase. As it turns out, there are rare human diseases where some of these proteins are mutated or absent, and in those states, people are highly susceptible to certain infections. The best characterized example is a deficiency in the ability to produce superoxide via phagocyte oxidase in a disorder known as chronic granulomatous disease http://www.nlm.nih.gov/medlineplus/ency/article/001239.htm. Thus, I would say that our own immune system effectively uses peroxide and bleach to kill bacteria and fungi.